Electro-optical signaling system



Patented Dec. 14, 1943 UNITED vSTATES PATENT orrics- ELECTRO-OPTICALSIGNALING SYSTEM Austin G. Cooley, New York, N. Y., assignor to TimesTelephoto Equipment Inc., New York, N. Y., a corporation of New YorkApplication February 11, 1942, Serial No. 430,349

(Cl. P18-7.1)

12 Claims.

This invention relates to signaling systems and more especially tosystems of the electro-optical type such as used in telefacsimile,telegraph and l fier, wherein the light source is excited fromcommercial supply lines and the level of the amplied output isautomatically adjusted in a predetermined ratio and is correlated to thedeparture of the said light source from anormal degree of brightness.

A feature of the invention relates to a power supply arrangement for alight source-photoelectric cell controlled amplifier, and having a novelform of voltage divider to compensate for abnormal variations in lightsource brightness.

Another feature relates to a system of recording signal conditionswherein one signal condition is represented by certain minimum waveamplitude and another signal condition is represented by a certainmaximum signal amplitude, and wherein discrimination of signals isdependent upon a predetermined ratio between said amplitudes, the systembeing controlled by a light source and photoelectric cell amplifier andhaving special compensating means to prevent background noise or thelike from interfering with the proper recording of the signal conditionseven though the power supply to the light source should depart fromanormal value.

A further feature relates to an improved line voltage compensatingarrangement for a photoelectric cell controlled amplier.

A further feature relates to a novel form of voltage divider.

A still further feature relates to the organization, arrangement andrelative proportioning of parts which cooperate to provide an improvedtelegraph signal recording. system such for example as a system forrecording marking and spacing signals.

Other features and advantages not speciiically enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claims.

While the inventive concept is capable of utility and is adapted to awide variety of signal transmitters, it ndsits primary utility insystems `of the type wherein the signals are of the marking and spacingtype such as used in tele-- graph, facsimile and like apparatus. In oneaspect the invention is in the nature of an improvement in the type ofsystem disclosed in application Serial No. 397,513, filed June 11, 1941,now Patent No. 2,328,056, granted August 31, 1943. Inasmuch as theinvention is concerned mainly with a photoelectric cell controlledampliiier and power supply therefor, only those parts of a telegraphsystem are shown and described as are necessary to teach one familiarwith the art how to practice the invention. Accordingly, in the drawing,

Fig. 1 is a composite schematic-circuit diagram embodying features ofthe invention.

Figs, 2 and 3 are Wave diagrams explanatoryA of the operation of Fig. l.

There is described in application Serial No. 397,513, led June 11, 1941,a system for recording facsimile subject matter in the form of black andwhite, e. g., ordinary printed or written matter. In such a system, theWhite" areas to be recorded are represented for example by signals abovea predetermined minimum amplitude; while the black areas are representedby the absence of signals or by signals below said minimum amplitude. Insuch a system, the signal discrimination between black and white is afunction of the ratio of the two signal amplitudes. One of the mainadvantages of such a "ratio system is that the ampliers can be adjustedso that the effects of the background noise in the amplifier orassociated equipment can be substantially eliminated or suppressed sobar as its effect on the recording is concerned. This is particularlytrue in those systems where the marking signal (e. g., a white area,signal) is only effective when the signal amplitude is above a certainminimum value. The maintenance of the proper minimum ratio between themark signal and the background noises or between the mark signal and thespace signal, if a space signal is being transmitted, is very importantin recording correct copy of black and White subject matter. Variousmethods of securing the desired ratio are known to the art, includingfor example many well-known methods of level and volume control in thesignal ampliiler. However, IA have found that even when theseconventional volume controls are employed, there is still a possibilityof false recording should the exciter lamp or its equivalent, vary inbrightness during signal recording. This'is especially true in the caseof photoelectric cell controlled amplifiers wherein the exciter lamp isenergized directly from the commercial supply lines which lines may besubject to voltage changes. Consequently with such known arrangements,if the level of the amplifier is adjusted for a given normal brightnessof the exciter lamp to produce a certain V. C. signal, the rate at whichthe A. V. C. signal -varies may not be sufllcient to compensate for therate of variation of brightness or the eilect of such variation ofbrightness on the recorded copy. In other words, the law of variation ofthe A. V. C. may not be properly correlated to the law of variation ofthe exciter lamp brightness versus supply voltage variation. I havedevised a novel form of level control and voltage divider for suchsystems whereby the rate or percentage of variation of the A. V. C. onthe output level of the amplifier can be matched or correlated to thevariations in exciter lamp brightness so that false recording of blacksand whites is substantially eliminated.

Referring to Fig.*1, there is represented bv the numeral I a subjectmatter to be transmitted, and merely for illustration this subjectmatter may be in the form of a tape or web on which the intelligence isprinted or written. The tapeor web is adapted to be scanned insuccessive elemental areas is as well-known inthe facsimile transmissionart, and for purposes Ofsimplicity, the scanning mechanism is omittedfrom the drawing. Associated with the scanning mechanism are' the usualexciter lamp`2 and the photoelectric cell 3 with their respectiveoptical systems l. I and 3. The electric signal from the cell 3 is inthe known way a function of the light and shade characteristics of thesuccessive elemental areas `oi the subject matter on tape I and thebrightness of lamp 2. The lamp 2 is steadily energized by the currentfrom the supply line 6 through transformer primary 'I and its step-downsecondary 8. The brightness of lamp 2 is therefore a function of theline voltage and is adjusted so that its normal brilliancy is such thatin cooperation with cell 3 it gives the required ratio I' of theresponse corresponding respectively to the black and white areas of thesubject matter. Preferably, the light which reaches the cell 3 isinterrupted by a rotating light chopper 3a whereby the response of thecell consists of a pulsating current of regular frequency having itsamplitude modulated' in accordance with the shade characteristics oi thesubject matter I.

The response of cell 3 is amplified in any suitable electron tubeamplifier which, in the drawing is shown as of the three-stage typewherein the amplifier tube 9 of the first stage is aresistance-condenser coupled to the amplifier tube I0 of the secondstage. The second stage is coupled through transformer I2 to the outputor push-pull stage I3. The output signal of stage I3 is then applied totransformer Il to control any well-known form of telegraph or facsimilereproducer either locally-or at a distance, it being understood that therecorder scans its recording surface in synchronism with the scanning ofsubject matter I as is well-known in the art.

The power for the various stages of the ampliiler and for the cell 3 isderived from line 6 through the power transformer and full-w averectifler I 5. 4'I'he rectified voltage is smoothed out by any suitablefilter comprising for example inductance I6 and shunt condensers I "I,I8. The resultant smoothed D. C. is applied across a voltage dividerwhich, according to the invention, consists of three sections in series.Two of these sections are constituted of resistors I9, 20, the voltagedrops across which vary with the total load current drawn by theamplifier; while the third section comprises a pair of gaseous typevoltage regulator tubes 2|, 22, of any well-known type, which have theproperty of maintaining a constant voltage drop thereacross forvariations `in the variable voltage on line 6.

For the purpose of supplying a steady D. C. potential to the anode ofcell 3, a potentiometer re`- sistance 23 is connected across tube 22 andthe adjustable arm 24 is connected to the anode of the cell. The high DC. voltages for the screengrid electrode 25 and for the plate electrodes26;-

21, 28 and 29, are taken of'f between the resistor I3 and tube 2|. Itwill be observed that the cathodes of the tubes are returned to groundthrough their respective grid bias resistors 30, 3I 32, and their shuntcondensers 33, 34 and 35, and that the junction between resistor 20 andtube 22 is likewise returned to ground. The control grids 33 and 31 ofpush-pull tube I3 are returned to ground through the arm 38 which is'adjustable along resistor 20. Consequently, the grid-bias voltage oftube I3 is a function of the normal plate current yof this tubewhichfiows through resistor'- ,32, and is also a function of the IR drop'I through the voltage divider resistance 20. I have found that byproperly proportioning the values .ofresistcrs` |13 -and 20, withrespect to the potential drop across tubes 2I and 22, it is possible tovmaintain lthe proper ratio betweenspace and mark signals in the outputof tube I3 so that the proper discrimination therebetween is maintainedat a-ll times and this discrimination is substantially independent of acomparatively wide change of. rightness in lamp 2resulting from a changeof voltage on line 6.

As one possible explanation of this compensation, reference may be hadto Figs. 2 and 3.

In Fig. 3, there are shown two signal conditions plotted against time,wherein-the high amplitude waves H represent the response of cell 3 whena white area of subject I is being scanned, while the low amplitudewaves L repre- -sent the response when a black area is being scanned. Inorder that there may be a proper discrimination in the record, it isnecessary that there be a definite minimum difference between the blackand white signals since otherwise the background noise or the like mayreach into the region of the white signal amplitude. Furthermore, if thelamp 2 should increase in brightness, it may cause the normal cellresponse c`orresponding to black to bel increased sufficiently to.

prevent proper discrimination between the black and white signals.

Since the recorder controlled by tube I3 must be rather sharplydiscriminating to the two amplitudes corresponding respectively to whiteand black, it is necessary that the ratio of the two signal amplitudesbe maintained constant even though the brightness of lamp 2 shouldaccidentally vary. This relation is illustrated in Fig. 2 wherein theabscissae may represent voltage applied to lamp 2 and the ordinates mayrepresent cell response. Theline OB represents the law of change of biason grid 36 with a change in line voltage, and line OB' represents thelawof change o! bias on grid 31 with change of line voltage. Thus, itwill be seen that as the line voltage drops, the cell response dropsbecause the brightness of lamp 2 is lowered. At the same time, thenegative bias applied to grids 36 and 31 through resistor 20 isdecreased in value, with a corresponding increase in the output of tubeI3. In Fig. 2, the signal at any given lamp voltage corresponding to ablack area is represented by the heavy black waves, while the signal fora white area is represented by the light or thin waves. Since for properrecording it is desirable to maintain a uniform fixed ratio between thewhite and black signal responses, the law of change of cell output forvariable line voltages, with respect to the law of change of grid biasmust be substantially uniform as represented by the lines OB--OC andOB'-OC. This can be effected by properly proportioning the resistors I9and 20 with respect to the voltage drop across tubes 2| and 22 and byadjustment of arm 38. As an illustrative example, it may be assumed thatthe plates of tube I3 are operated at a normal plate voltage of 300volts;

that the system is designed to operate with a normal voltage on the line6 of 110 volts, and that for this condition the total voltage developedacross i9, 2i, 22 and 2, is 350 volts. Then the resistors I9 and 29 canbe proportioned so that the drop across resistor. i9 is for example 30volts and the drop across resistor 2li is 20 volts, and thereforethereis a steady voltage across tubes 2l, 22 of 300 volts. Under thiscondition, a maximum of 2O volts negative bias can be applied to grids36 and 31 by arm 38. If the line voltage should drop so that the totalvoltage across the voltage divider is only 325 volts, the drop acrosstubes 2l and 22 will still be 300 volts, leaving a balance of 25 voltsto be divided between resistors i9 and 2G. By proper proportioning of'these two resistors therefore, the necessary law of change in dropacross resistor 2@ to compensate properly for the law of change inbrightness of lamp t can be achieved. The I same applies if the linevoltage should rise above llO volts.

It will be understood that if desired, the total grid bias applied togrids 3S and 3l may be such that the respective plate circuits of tubei3 pass current only when the marking or white signal amplitude H (Fig.3) is applied. Consequently, the recording device will be operated toreproduce a mark signal or to reproduce a white area only in the regionwhen the amplified cell response fis between the lines OB and OC (Fig.2). Consequently, the bias OB can be automatically adjusted so as toeliminate from the output of tube I3 the background noise which would berepresented by the area between OO' and OB or between OO' and OB'.

While in the foregoing description, reference has been made to white andblack areas, it will be understood that these are usedf in a relativesense and would include respectively dark and light areas, or areaswhich have a sufllcient contrast in shade to be considered different forthe purpose of signal production and transmission. v

While one specific embodiment has been disclosed, it will be understoodthat various changes and modiiications may be made therein withoutdeparting from the spirit and scope of the invention.

What I claim is:

l. In a system of the type employing an electron tube ampliiiercontrolled by a light sourcephotoelectric cell combination, a source ofvoltage for energizing the light source, -a voltage divider energizedfrom said voltage source and connected to the electrodes of saidamplifier, said voltage divider comprising a constant voltage sectionconnected in series with a variable voltage section, and a connectionfrom said variable voltage section to the control grid of said amplierto vary the bias of said control grid in a predetermined laW withrespect to the law of change in brightness of said light source when theline voltage changes.

2,. In a system of the type employing an electron tube ampliercontrolled by alight sourcephotoelectriccell combination, a source ofvoltage for energizing the light source, a voltage divider energizedfrom said voltage source and connected to the electrodes of saidamplier, said voltage divider comprising a pair of resistors whose IRdrop varies in accordance with the load current of the-amplifier and inseries with said resistors a 'constant voltage device, means forapplying the constant voltage across said device to the output electrodeof said amplier, and means to bias the grid of said amplier from thevoltage developed across one of said resistors.

' 3. In a signaling system of the type employing an electron tubeampliiier controlled by an exciter lamp and photoelectric cellcombination, a source of voltage for said lamp which source is subjectto undesirable voltage variations, a D. C. voltage divider energizedfrom said source, and having one section constituted of a variable IRdrop resistor and another section constituted of at least one gaseoustypevoltage regulator tube, a connection from said voltage regulatortube for supplying a constant D. C. voltage to an output electrode ofsaid amplier, a connection from said resistor to the input controlelectrode of said amplifier, said resistor being proportioned so thatthe negative bias applied to said input electrode varies automaticallyin direct ratio to the varia# tion of brightness of said lamp.

4.. A system according to claim 3 in which said amplier tube is alsoprovided with a self-biassing arrangement whereby its grid isself-biassed negatively in proportion to the average plate currentflowing therethrough.

5. In a signaling system of the type employing an electron tubeamplifier controlled by an exciter lamp and photoelectric cellcombination, a power vsupply for said lamp, a self-biasing arrangementfor the grid of said tube whereby said grid is negatively biassed inaccordance with the average plate current of said tube owingtherethrough and substantially independent of a wide range of variationsin said power supply source, and another biassing arrangement for thegrid of said tube wherebysaid grid is negatively biassed in proportiontoa change in brightness of said lamp.

6. In a signaling system of the type employing an electron tubeamplifier controlled by an exciter lamp and photoelectric cellcombination, a power supply circuit for said amplifier, said powersupply circuit being fed from a supply line which is subject toundesirable voltage variation, means to energize said lamp substantiallydirectly from said supply line, means to supply D. C. power derived fromsaid line to said amplifier, the lastmentioned means including a voltagedivider comprising at least one gaseous type voltage regulator tubeconnected in series with a variable IR. drop resistor and with the pointof connection between said regulator tube and said resistor connected toground, means to connect the output electrodes of said amplifier acrosssaid regulator tube, and means to connect the control grid oi' saidamplier tube to a point on said resistor whereby the total negative biason said control grid varies in constant ratio to the variation inbrightness of said lamp.

7. In a signaling system or the type wherein signals of two differentamplitudes are transmitted to reproduce respectively two differentsignal conditions, e. g., marking and spacing, an exciter lamp, aphotoelectric cell and means for varying the excitation of said cellfrom said lamp to produce said two signal conditions, an electron tubeampiiner for said cell, and means to maintain a constant ratio betweenthe two signal conditions in the output of said ampliiier over acomparatively wide range of variation in brightness of said lamp, thelast-mentioned means including a voltage supply for the ampliiier havinga section which is stabilized to remain uniform in voltage underdifferent amplifier loads and another section which varies in voltage inaccordance with the amplifier load, and means to adjustably connect saidother section to the control grid of said amplier tube.

8. A system according to claim 7 in which a potentiometer resistance isconnected across said stabilized section, and a connection isprovidedfrom said potentiometer resistance to the anode of said cell.

9. A system according to claim 7 in which said amplifier tube is alsoprovided with a grid selfbiassing circuit which applies a negative gridbias to the grid in proportion to the average output current oi saidtube iiowing therethrough.l

10. In combination a lamp, a power Supply line connected to said lamp, avoltage divider, a rectifier and filter connected between said line andsaid voltage divider, said voltage divider having two variable IR dropresistors connected in series with a gaseous type voltage regulatortube, at least one of said resistors being connected between the saidregulator tube and the negative side of the rectiier, a voltagetapbetween said regulator tube and the positive side of the rectier, anda ground connection on the negative.

side of said voltage regulator tube.

11. In combination, a photoelectric cell, an electron tube amplifier forsaid cell, a power supply circuit for said cell and ampliiier, a voltagedivider across said circuit, said voltage divider comprising a pair ofgaseous type voltage regulator tubes connected in series with aresistance, means to connect the plate-cathode circuits of saidamplifier across said pair of regulator tubes, means to connect thecontrol grid-cathode circuit of said amplifier tube across saidresistance, and means to connect the anode-cathode circuit of said cellacross one of said regulator tubes.

12. In a system oi.' the type employing an electron tube amplifiercontrolled by the combination of a photoelectric cell and a light sourcefed from a supply voltage, means to compensate for undesirablevariations in brightness of the lightsource comprising grid bias meansfor the amplier tube, and means to control said bias means jointly inaccordance with the brightness of said light source and in accordancewith the voltage applied to said light source.

AUSTIN G. COOLEY.

